Known clinical analyzers are employed for testing a large number of sample fluids for a particular analyte(s) of interest. In analyzers having so-called wet chemistry systems, sample fluid is typically placed in a sample reaction vessel, such as a cup or tube, in the analyzer so that aliquots can be dispensed or metered. Structurally within the analyzer, a probe or proboscis using appropriate fluidics such as pumps, valves, liquid transfer lines such as pipes or tubing, and driven by, pressure or vacuum are often used to meter and transfer a predetermined quantity of sample from the sample vessel to a reaction vessel. The sample probe or proboscis or a different probe or proboscis is also required to deliver diluent to the reaction vessel, particularly where a relatively large amount of analyte is expected or found in the sample. A wash solution and process are generally needed to clean a nondisposable metering probe. Here to, fluidics are necessary in order to accurately meter and deliver wash solutions and diluents.
In addition to sample preparation and delivery, the action taken on the sample requiring measurement often requires dispensing of at least one reagent, substrate, or other substance that combines with the sample to produce some noticeable event such as fluorescence, absorbance of light or chemiluminescence. Several different substances are frequently combined with the sample in order to attain the detectable event. This is particularly true with immunoassays since they often require multiple reagents and wash steps. Reagent metering systems are used in order to accomplish the above tasks. Generally, these metering systems also require a wash process in order to avoid carryover. Therefore, these systems also include fluidics in order to expedite the cleaning of a probe or proboscis and the addition of a signal reagent that reacts with bound analytes to produce a chemiluminescent effect.
A device located within the analyzer, such as a luminometer, can then be used to determine the appropriate measured quantity, such as light absorbance by the bound sample.
The sample and reagent delivery systems often require the accurate and very precise transport of small volumes of liquids. Therefore and in certain prior art clinical analyzers, redundant detection schemes have been introduced for detecting incorrect volumes of fluid which are being delivered to a reaction well or vessel by the various fluidic metering subsystems of the analyzer. The intent of these detection schemes is to suppress incorrect results that occur from either too much fluid or too little fluid being delivered into a reaction well. Incorrect results can greatly influence the efficacy of testing, particularly in systems utilizing very small metering volumes. According to one exemplary indirect detection scheme, measurements are taken between reagent metering events rather than by measuring fluid levels directly in the reaction well.
For example and in the case of the metering of signal reagent (hereinafter referred throughout as “SR”), a nominal fluid volume that is delivered to the reaction well or vessel is approximately 200 microliters, while a typical fluid volume of sample and reagent (hereinafter referred to as S+R) which can include a fluid volume of the combination of reagent A and/or reagent B and patient sample is approximately 100 to 180 microliters.
For purposes of measurement, a capacitance or other type of sensing technique, is usually employed. In brief, a well wash probe is lowered to the reaction vessel via manipulation of a stepper motor with each step being counted by a data processing system. In a preferred embodiment, vertical drive resolution is approximately 39 steps per mm. The lowering process continues until the tip of the well wash probe which contains a capacitance sensor, contacts fluid. The height of the descent is then converted into a volumetric measurement of the total of fluid (sample, reagent, sample+reagent, well wash and signal reagent) via a formula, or by reference to a table or database made from a formula or derived empirically. A suitable formula, for example, is readily derived by empirically establishing the proper height, graphically depicting the descent of the well wash probe until contact is made in a vessel (according to scale) using a commercial CAD software tool, and querying the CAD program to produce the equation that converts the height to volume.
In terms of an error budget to ascertain fluid volumes, a major portion of this budget is accounted for by the geometry of reaction well; that is, a significant contributor is whether the reaction well is large or small in geometry as a result of well manufacture. For example, and referring to FIG. 1, a nominal reaction vessel or well 135 is compared to typical large and small vessels 135a, 135b, respectively. Using the above capacitance sensing technique, a large reaction vessel 135a, due to its larger overall size and depth, will indicate an apparent decrease in the amount of fluid volume sensed since the volume measurement is based on the height of the fluid present in the vessel. Similarly, a small reaction vessel 135b will indicate an apparent increase in fluid volume as compared to the nominal reaction well 135, in spite of the fact that each of the above vessels, in reality, have each had a nominal fluid volume dispensed. The above discrepancies between large and small wells are due to manufacturability, but it will be apparent that the ability to accurately screen for well geometry is essential, especially when small or microvolumes on the order of those noted above, are being dispensed.
If analytical test capability is to be enhanced, a testing strategy and mathematical algorithm needs to be developed to remove well geometry effects as well as those caused by other metering specific sources, from the total error budget. The above reasoning is true given that the volume contained in the reaction vessel is inferred from well height as opposed to a nominal well geometry. As noted, an internal well geometry that is large using commonly known capacitance sensing techniques to detect the volume will appear to have a decreased volume of fluid associated with the lower measured fluid height. Conversely, a small well geometry using the same measurement capacitance measurement technique will appear to have an increased volume of fluid associated with a higher fluid height as shown in FIG. 1. In addition, consideration should be made to other factors such as the meniscus formed at the top of the fluid column, as shown in FIG. 1, the extent of which varies depending not only on height of the fluid but also the properties of the fluid, (Signal reagent, sample, etc.) in the reaction vessel. A consideration which also contributes to the error budget is whether the probe is centered, as an off-center detection measurement will be affected by the above meniscus. Still another factor that is well specific involves the coatings applied thereto to bind analytes.
In terms of attempting to remove well geometry as a significant contributor to the overall error budget, gravimetric measurements of reaction wells could be measured both prior to and after fluid delivery can remove reaction well geometry as a contributor to the overall analytical error which is described above. However, this particular approach adds significant complexity and cost to the analyzer, as well as increasing the potential of degrading throughput and overall system reliability thereof.
Another alternate solution would be to significantly tighten the dimensional specifications of the reaction well(s). This solution, however, would be extremely costly given that the manufacturing molds for reaction vessels are high yield, multi-cavity and have a material/wear issue based on the use of titanium dioxide which is abrasive and causes mold wear. That is, mold maintenance and process control would increase while mold life would decrease, adding significant cost and complexity to the manufacturing of reaction wells.